1. Field of the Invention
This invention relates to electron multiplier devices and, in particular, to electron multiplier devices for detection of particles emitted from a surface as a result of an incident beam impacting the surface.
2. Related Art
Electron multipliers are useful tools for various applications, including the detection of photons, electrons, ions and heavy particles. Such detectors are utilized in various spectroscopic techniques, including Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and electron energy loss spectroscopy. Further, electron multipliers may be utilized for detection of secondary and back-scattered electrons in scanning electron microscopes, focused ion-beam tools, or e-beam lithography tools.
In general, electron multipliers have had two configurations, the channeltron multiplier or multi-channel plate multiplier. FIG. 1 shows a parallel plate electron multiplier 100 as described in numerous publications, including S. Suzuki and T. Konno, xe2x80x9cA Computer Simulation Study On the Detection Efficiencies of Parallel-Plate Electron Multipliers,xe2x80x9d Sci. Instrum. 66 (6), p. 3483-87 (June, 1995); and L. P. Andersson, E. Grusell and S. Berg, xe2x80x9cThe Parallel-Plate Electron Multiplier,xe2x80x9d J. Phys. E: Sci. Instrum., Vol. 12, p. 1015-22 (1979).
Electron multiplier 100 includes secondary emitting surfaces 101 and 102, deposited on glass plates 111 and 112, respectively, and separated by a spacing 104. A voltage Vd is applied along the length of electron multiplier 100 so that electrons entering at an open end 105 are accelerated along the length of electron multiplier 100 away from open end 105. When the electron collides with one of secondary emitting surfaces 101 and 102, multiple secondary electrons are emitted. The secondary electrons are then accelerated along electron multiplier 100 and themselves may collide with one of secondary emitting surfaces 101 and 102. On each collision of an electron with sufficient kinetic energy with one of emitting surfaces 101 or 102, further electrons are emitted. By repeated collisions of electrons with secondary emitting surfaces 101 and 102, an output pulse containing a very large number of electrons is emitted from electron multiplier 100.
The output pulse is received by collector 103 located on the side of electron multiplier 100 opposite from open end 105. Typically, collector 103 is held at an elevated voltage from the voltage of that end of electron multiplier 100. The output pulse is detected by detection circuitry 106 coupled to collector 103. The gain of electron multiplier 100 depends on the voltage Vd applied across electron multiplier 100, the secondary emission properties of secondary emitting surfaces 101 and 102, and the physical dimensions of electron multiplier 100.
In some electron multipliers such as electron multiplier 100, further voltages are applied to either end of one of secondary emitting surfaces 101 and 102. In such cases, electric fields can be created that are not parallel with the length of electron multiplier 100, thereby enhancing collisions with one of secondary emitting surfaces 101 or 102. Further, collector 103 may be tilted (i.e., the collector surface may not be perpendicular to the surfaces of secondary emitters 101 and 102) in order to further enhance collection of output pulses of electrons and to further supply a component of the electric field not parallel with electron multiplier 100.
Some electron multipliers may be constructed from a glass tube coated with a secondary emitting surface. The resulting multiplier, in principle, operates as described above for parallel plate electron multiplier 100 except that, instead of parallel plate secondary emission surfaces, the secondary emission surface is cylindrical in shape. The tubular channeltron multiplier has the advantage that, because of its tubular nature, it can be shaped into loops and spirals that reduce its overall size without affecting the overall length of the multiplier.
However, each of these multipliers are difficult to use in certain environments. For example, in some instances, such as in lithography or in electron microscopes, it is difficult to detect reflected electrons that are close to an incident electron beam. In some applications, it is desirable to collect electrons from as close to the incident beam as possible. With electron multiplier 100 or the tubular channeltron multiplier, positioning of the opening surface can be difficult.
Therefore, there is a need for an electron multiplier that is easily constructed, of small size, and capable of monitoring the particles close to a beam incident on a surface that emanate from the surface.
According to the present invention, an electron multiplier capable of detecting particles such as, for example, ions, photons, or electrons, traveling close to an incident beam is presented. The electron multiplier includes a top plate and a bottom plate separated by a small gap. Each of the top plate and the bottom plate includes an access through which an incident beam can pass. The accesses of the top plate and the bottom plate are aligned so that the incident beam can pass through the electron multiplier. Particles traveling close to the incident beam, and in a direction opposite that of propagation of the incident beam, can enter the electron multiplier between the top plate and the bottom plate and thereby be detected.
The top and bottom plates each have a secondary electron emitting surface. The secondary electron emitting surface of each of the top and bottom plate emit electrons when the surface is impacted with a particle of sufficient energy. Further, each of the top and bottom plates are resistive so that a current can flow through them. Finally, in most embodiments, the top and bottom plates provide structural support for the secondary emitting surfaces.
In some embodiments, the top and bottom plates can be a single material, for example lead oxide glass, bismuth oxide glass, or iron borate glass. These materials are resistive, provide a secondary emitting surface, and provide structural support. In another embodiment, each of the top and bottom plates can include secondary electron emission layers, for example CVD diamond or an alkali halide, deposited on a resistive layer, for example a metal or low resistance semiconducting layer, deposited on a structural substrate, such as glass.
In one embodiment, the top plate and the bottom plate have an annular geometry. The top secondary emitting surface has an outside radius and an access with an inside radius, and the bottom secondary surface has an outside radius and an access with an inside radius. In another embodiment, the secondary electron emitting surface of the top and bottom plates have annular geometry. The access allows an incident beam to pass through the top plate, with the top secondary emitting surface, and the bottom plate, with the bottom secondary emitting surface, without impacting the electron multiplier.
The references to top and bottom or up and down in this disclosure is with reference to the direction of propagation of an incident beam. Bottom or down refers to a direction closest to a surface on which the incident beam is incident. Top or up refers to the opposite direction from bottom or down.
In most embodiments the outside radius of the top secondary emitting surface and the outside radius of the bottom secondary emitting surface are about the same. In some embodiments the inside radius of the access of the top is less than the inside radius of the access of the bottom secondary emitting surface so that particles (e.g., electrons, ions, or photons) are easily captured into the multiplier.
The access through the annular geometry, through which a beam can pass, has the inside radius of the top secondary emitting surface at the top plate and the inside radius of the bottom secondary emitting surface at the bottom plate of the electron multiplier. In most embodiments, the access is arranged to be at the center of the annular geometry of the electron multiplier.
Voltages applied to electrodes coupled to the top plate and the bottom plate at the inside and outside radiuses of the top secondary emitting surface and the bottom secondary emitting surface provide a radial electric field in the annular electron multiplier. A collector is arranged around the outside radius of the top secondary emitting surface and the bottom secondary emitting surface to collect any burst of electrons emitted from the electron multiplier.
In some embodiments of the invention, the annular collector is segmented. Each segment of the annular collector is coupled to detection electronics in order to measure the angular distribution of particles received into the multiplier. Further, in some embodiments the secondary electron emitting surfaces of the top and bottom plates are segmented in order to enhance the measurements of the azimuthal distribution. Embodiments that segment the secondary electron emitters may have the advantage of reducing distortions of measurements of the angular distribution.
In some embodiments of the invention, the electron multiplier includes several stacked plates, each with secondary electron emitting surfaces, each separated by a small distance. Each of the stacked plates has an access with a different inner radius and are arranged in order of the size of the inner radius. In most embodiments, the plate with the largest inner radius is on the bottom (i.e., closest to the surface on which the incident beam impacts) and the plate with the smallest inner radius is on the top. Plates having inner radiuses of various sizes are arranged accordingly between the top plate and the bottom plate. Embodiments of this type provide the ability to measure the angular distribution of particles emanating from the surface. Those particles having the smallest angle from the incident beam impact on the top secondary emitting surface while those particles with larger angles impact on other secondary emitting surfaces of the stacked plates in the electron multiplier.
In some embodiments of the invention, the resistivity of the plates can be varied radially in order to affect the electric field. Further embodiments may radially adjust the separation between plates, causing the secondary electron emitting surfaces to be, for example, conically or terraced shaped. Further, the electric field may be tilted (i.e., have components perpendicular to the radial direction) by adjusting the voltages supplied to the various electrodes and the bias voltage of the annular collector and tilt of the annular collector.
These embodiments, along with others, are further discussed below with reference to the following figures.